It's probably your shell.

If you're using zsh:

> When a partial line is preserved, by default you will see an inverse+bold character at the end of the partial line: a ‘%’ for a normal user or a ‘#’ for root. If set, the shell parameter PROMPT_EOL_MARK can be used to customize how the end of partial lines are shown.

See http://zsh.sourceforge.net/Doc/Release/Options.html#Promptin...

I got `2, 1, 0,`

I read the instruction as: "Set i to 3. Subtract 1 then print until false."

    i = 3
    3 - 1 = 2, print
    2 - 1 = 1, print
    1 - 1 = 0, print
    0 = false, stop

I see a lot of people complain about using `--` like that but I fail to see what is unintuitive about it if you actually read the code and see what i is initialized as. =\

Ugh, never mind. I was misinterpreting that the shell was saying. See? Told you I'm dumb.